lehmann_black_cube_linear_usb_headphone amplifier review

8/7/2019 Lehmann_Black_Cube_Linear_USB_Headphone Amplifier Review

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EQUIPMENTREVIEW

22 | Australian Hi-Fi

LehmannaudioBlack Cube Linear USB Headphone Amplifier

Norbert Lehmann is a highly skilled

and very talented engineer who operates

from what must be the world’s best address

for an audio company: Mozartstrabe, Koln,

Germany. I do wish, however, that he didn’t

have such a penchant for naming his prod-

ucts ‘Black Cube.’ At least the original ‘Black

Cube’ was black, even though it wasn’t

a cube, but this latest ‘Black Cube’ is neither

black nor a cube!

Even its name is not completely descrip-

tive because, in addition to being a head-

phone amplifier, and a DAC that will con-

vert your digital audio files to analogue (via

a USB interface), the Black Cube Linear is also

a high-end preamplifier (albeit one that has

only a single input, so if you want to accessmultiple signal sources, you’ll have to add an

outboard passive switch.)

The EquipmentThe Black Cube Linear USB Headphone Am-

plifier/Single-Source Preamplifier (which here-

after I will refer to as the Black Cube Linear,

rather than continue to use its full title, for

the obvious reason) is surprisingly large and

heavy. One reason is that Lehmann has de-

cided to integrate the power supply into the

case, whereas with many of his other designs,

he provides a completely separate power sup-

ply. I suspect there are a number of reasons

for this, not least being the fact that recent

improvements in power supply technology

now mean that it’s possible (though by no

means easy, and certainly expensive!) to build

a power supply that’s quiet enough to be in

close proximity to low-level voltages without

introducing noise.

Another reason for designing an internal

power supply is no doubt the popularity of

Lehmann’s products in the professional audio

field, where almost everyone uses headphones

whilst monitoring, and thus has need not

only of state-of-the-art headphones, but also a

state-of-the-art headphone amplifier to drive

them! (About which more later…) In a profes-

sional situation, where you have equipment

strewn everywhere about the studio, you just

don’t want a two-box solution, particularly if

the power supply has been separated from the

low-voltage electronics because it’s noisy! A

professional would immediately realise that if the Black Cube Linear’s power supply is quiet

enough to be contained within the case, the

Black Cube Linear itself won’t introduce any

noise into the recording chain.

You really only need take a passing glance

at the Black Cube Linear to see immediately

that it’s very obviously been built by an en-

gineer. Who else would be game enough to

have the very large volume control knob pro-

trude through an equally large hole on the

front panel with such a fine tolerance? Who

else wouldn’t care that in order to attach such

a large-sized knob (which makes the volume

so easy to adjust in tiny, yet precise incre-

ments) a large grub-screw hole would need to

be clearly visible on the upperside when the

volume is set between 7 o’clock and 9 o’clock

and also between 3 o’clock and 5 o’clock?

And who else would have made the chassis

entirely out of 2mm steel and the front panel

from 4mm thick anodised aluminium, then

held everything together with stainless steel

and blackened high-tensile steel nuts and

bolts, all with hex-heads?

Flip the Black Cube Linear upside down

and you’ll see even more ‘Engineer At Work’

signs in the form of two pairs of DIP switches

(used to adjust the gain individually for each

channel between 0dB, 10dB and 20dB) and

the fact that the operating instructions (as well

as the labelling for the rear panel input and

output terminals) is also underneath, neatly

printed on aluminium ‘stick-on’ patches.

I’d judged the Black Cube Linear a work

of art even before I pulled out my trusty hexkey to remove the ten bolts that would give

me access to the secrets of the interior but ten

screws and 100 turns later, I was even more

impressed. Firstly, the two 6.5mm headphone

jacks on the front panel that appeared to be

black plastic actually turned out to be Neu-

trik’s top-line gold-plated phone jacks. Id

already picked the volume control as one of

Alps’ finest, just from the perfectly symmet-

rical rotation and the way it glided around

freely as I turned it, yet stopped without

any backlash at any point I released it, but

it was nice to have my hunch confirmed.

Then there’s the shielded Sedlbauer Poly-

tronik 30VA toroidal transformer that feeds

the two Vishay 4,700μF/40V capacitors and

discrete rectifier diodes. Although the left and

right channels share the same power supply,



Australian Hi-Fi | 23

Brand: Lehmannaudio

Model: Black Cube Linear USB

Category: Preamplifier/Headphone Amp/DAC

RRP: $1,849 ($1,449 w/o USB)

Warranty: Two Years

Distributor: Syntec International Pty Ltd

Address: 60 Gibbes Street

ChatswoodNSW 2067

T: 1800 648 628

T: (02) 9910 6700

F: (02) 9910 6710

E: [email protected]

W: www.syntec.com.au

Lehmannaudio Black Cube LinearUSB Headphone Amplifier

Why Use A HeadphoneAmplifier?When considering why you’d want to use

a headphone amplifier, first ask yourself

whether you’d consider connecting a pair of

speakers rated at 50-ohms to your amplifier,

the Owner’s Manual for which more than

likely suggests you connect speakers with

a nominal impedance of between 4-ohms

and 8-ohms, or maybe, at a stretch, 16-ohms.

Most likely you wouldn’t even contemplate

connecting a pair of 50-ohm speakers to your

amplifier, yet if you connect a pair of head-

phones to the headphone socket on your am-

plifier, you could be looking at a ‘mismatch’ of

equal—or even greater—severity. Some head-

phones require the headphone output circuit

to produce almost seven volts to give accept-

ably loud reproduction. This could mean

power consumption of almost 3-watts. Few (if

any) of the standard headphone circuits fitted

to amplifiers or receivers will deliver either

of these extremes. Then there’s simply the

question of the quality of the audio path. If

an amplifier manufacturer is going to try to

cut costs, the headphone circuit is always the

first one on the chopping block. Manufactur-

ers that really care about audio quality often

remove the headphone output entirely rather

than provide an inferior one. Others just put

in the cheapest ‘off-the-shelf’ circuit they

can and hope that their customers aren’t re-

ally that interested in listening to their music

via headphones.

So far, my discourse has presumed you’re

not listening to music stored on a computer.

If you are trying to listen to computer mu-

sic files, the problem is magnified even more,

because not only is the computer headphone

output not optimised for quality headphones

(and the headphone amplifier inside the com-

puter the cheapest possible that will do the

job), but also the quality of the audio signal

is compromised (that’s actually the technical

term: I’d prefer to say ‘completely destroyed!’)

by the digital ‘hash’ generated by the circuitry

inside the computer.

If you’d like a full and detailed explana-

tion of all the issues, I’d suggest you should

read the truly excellent and very accessible

‘White Paper’ on the subject published by

Norbert Lehmann. You can find it at www.le-

hmannaudio.com/download/info/pdf_print/

EN_techletter_headphone_amplifiers.pdf

Use and Listening SessionsHeadphone devotees will know that head-

phones are ruthlessly revealing of many audio

nasties, but the one they are best at revealing

is high-frequency hiss, because hiss that’s of-

ten completely inaudible when listening via

speakers can be made cruelly audible when

listening via headphones. So the first thing I

did with the Black Cube Linear was to check

for hiss, simply by plugging my headphones

into the front-panel socket and then, with

a CD paused, turning the volume up to maxi-

mum. I couldn’t hear anything. Not a sausage.

This is one quiet headphone amplifier! I then

repeated the exercise using the USB input.

Again, nothing. Zip. Zero. Nada. I couldn’t

everything from there on is completely dual

mono—though of course they’re contained

on the one PCB. All amplification is via pre

Class-A, with no global feedback applied. The

DAC is a Burr-Brown PCM2702E.

The rear panel is equipped with a stand-

ard 3-pin IEC mains socket with integral fuse,

four gold-plated RCA terminals (two of which

are unbalanced line inputs, and the other two

unbalanced line outputs), a USB connector,

and a 3.5mm stereo phone socket (so you can

connect an iPod). This 3.5mm stereo socket

is just paralleled with the RCA inputs, so you

can’t use both at the same time. There are also

two tell-tale LEDs on the rear panel: one red,

the other green. These show the status of the

USB connection. If the Black Cube Linear de-tects a valid data stream coming in via USB,

the green LED will light. If the red LED glows,

it means the Black Cube Linear can’t make

a link with the computer. If both LEDs remain

off, it means the Black Cube Linear has made

a valid USB connection with the computer,

but there’s no audio coming down the USB

pipe. If you have both the USB and the ana-

logue inputs connected, the Lehmann will al-

ways auto-switch to the active input. If both

inputs are active, the Lehman always gives

priority to the USB input.

As you may be able to tell from one or

more of the photographs accompanying

this review, the Black Cube Linear’s case is

disproportionately deep. The specifications

say 280mm, which would be deep enough,

but that’s just the chassis. If you include the

gold-plated RCA terminals on the back, the

depth increases to around 290mm, and if you

add in the volume control, it becomes almost

315mm. The chassis is 118mm wide at the

front panel (though only 114mm across the

chassis itself, back from the front panel) and

stands 52mm high on its four rubber feet. The

unit weighs 1.5kg.

“Manufacturers that really care about audio quality

often remove the headphone output entirely”



LAB REPORT

Xxxxxxxxxxxxxxx


I also picked up ‘pre-loved’, though not at

a garage sale). It’s difficult to describe ‘better’

because the Perreaux is no slouch, so I’m not

talking chalk and cheese here, but I thought

their sound through the Black Cube Lin-

ear was clearly smoother and more relaxed,

with a slightly more urgent sense to the bass.The only area where I thought the two level-

pegged was the treble, where both allowed

the Beyers to give their typically pure, silky-

sounding best.

Then, equally deliberately, I listened using

my Koss r/80 headphones, purchased brand-

new without the benefit of a reasonable

audition, or even having read an independ-

ent review (yes, I know, I know…). The two

things the Koss headphones do have going

for them is that they’re very comfortable and

that if I ever jump up and down on them in

a fit of rage for having purchased the things

in the first place, Koss will happily replace

them for free! I’d like to be able to say there

was a fairy-tale ending, as in a frog turning

into a princess, and that the Black Cube Lin-

ear transformed their sound from something

quite bass-heavy and dull-sounding into taut,

tight, bass and crystal clarity at the top end

but alas, it was not to be. It appears that even

the Black Cube Linear cannot turn a sow’s ear

into a silk purse. Which is not to say there

was not an improvement. I thought the r/80s

sounded more dynamic than usual, and the

bass, although still too heavy for my liking,

was more clearly delineated, making it easierto follow the bass lines.

I thought I would wrap up my review by

evaluating the Black Cube Linear with my

usual ear-warmers, which are my much-loved

and much-used Sennheiser HD650s, which

are simply stunning-sounding headphones. If

you can ever get to listen to a pair of these,

I’d recommend you do, even though they will

spoil you for anything else. It also helps that

they’re so superbly comfortable that you can

easily forget they’re there—but also means

you need to be careful because at the price,

you wouldn’t want to forget you’re wearing

them and walk away, running the risk of dam-aging them when the cord runs out! Their liq-

uid midrange sound was made even smoother

and more fluid by the Black Cube Linear, and

the music seemed to have more ‘space’ around

the notes, so that the sound was effortless.

I’d always appreciated the HD650’s dynam-

ics, but the Black Cube Linear took them to

the next level, improving both the ‘attack’

of transients, but also imbuing the recovery

period with equal speed. It meant that even

when I was playing quite loudly, there was

never any sense of the music being loud: just

of the music being powerful and dynamic.

Almost wrapped up, I should have said. I’d

been so carried away by the Black Cube Lin-

ear’s performance in a conventional analogue

chain that I’d completely forgotten its USB

capability. I am still in the process of trans-

ferring my CD collection to hard disk, not so

much because I plan to listen to them played

back from hard disk (at least not so far as anyserious listening is concerned!) but so that in

the event of a burglary or a house fire, I will

at least have a copy of my music collection

(the sum of twenty years of serious collecting

and a record of my changing musical tastes)

tucked away somewhere safe. Listening to

some of my favourites via the Black Cube Lin-

ear I can see that at some point in the future,

I might have to consider the possibility of se-

rious listening via a computer drive, but on

the current evidence, that time is yet to come.

I should note at this point that you can buy

a Black Cube Linear without USB (not surpris-

ingly known just as the Black Cube Linear),

for just $1,449, which is a not-insignificant

saving if you think you’re really not going to

get the use from the USB circuit.

Conclusion

If you are starting out to put together a high-

end audiophile system, the Black Cube Linear

USB is an absolute bargain, because in one fell

swoop (and for just $1,849!) you’re scoring

a high-end preamp, a high-end headphone

amplifier and a high-end USB audio interface.

So all you have to do is add a CD player, a pow-

er amp and a pair of speakers and you’re away,with a system that will be the envy of all!

Chris Croft

Readers interested in a full technical

appraisal of the performance of the

Lehmann Audio Black Cube Linear

USB should continue on and read

the LABORATORY REPORT published

on the following pages. All readers

should note that the results men-tioned in the report, tabulated in per-

formance charts and/or displayed

using graphs and/or photographs

should be construed as applying

only to the specific sample tested.

hear hiss or hum. I was very surprised that

the USB input didn’t even buzz the tiniest bit,

because I was expecting at least something,

though I was very happy to have my expecta-

tions shattered in such a pleasant way.

It wasn’t until I started listening to the

Black Cube Linear that I discovered the dif-ference between the two headphone sockets.

Plug into the left-most socket and the signal

from your loudspeakers will instantly cease

(assuming that you have wired the Black

Cube Linear so that it’s in the source line-in-

put, or in a pre/power amplifier loop). Plug

into the right-most socket and you can listen

to both the headphones and your speakers

simultaneously. Why would you want to do

this? I often do if I’m reading and listening to

music in the background, because my favour-

ite reading position isn’t in the ‘sweet spot’

between the speakers, so by wearing head-

phones, I get a perfect stereo image, and the

bass from the speakers is ‘depthier’ than I get

from my headphones, resulting in a far more

immersive experience, particularly when I am

playing at very low volume levels… as I usu-

ally am when I am reading. You can use both

outputs simultaneously, to drive two pairs of

headphones, but unless the headphones are

identical, the volume level will be different in

each pair. Even if the headphones are iden-

tical, so the volume levels are the same, it’s

unlikely any two people would want to listen

at the same volume in any case. Still, I’m sure

it’s handy to be able to accommodate a sec-ond listener from time to time…

I started listening to the Black Cube Lin-

ear with a pair of open-back headphones

also made in Germany: Beyer DT990s. These

aren’t exactly a huge favourite of mine, but

they’re so famous that I couldn’t resist pick-

ing up a damaged pair for $5 at a garage sale,

taking advantage of the fact that the seller

obviously didn’t know what they’d cost re-

tail, and that unlike many headphones, you

can get spare parts for this model, so I was

easily able to restore them to ‘as new’ condi-

tion. As it happened it proved to be a good

choice to start, because they sounded muchbetter through the Black Cube Linear than I’d

ever heard them sounding before, and that

includes through my own headphone am-

plifier, the Perreaux Silhouette SXH2 (which

Lehmannaudio Black Cube Linear USB Headphone Amplifier



TEST RESUL TS

Test Results

The frequency response of the Black Cube

Linear proved to be highly linear, and very ex-

tended, reaching up to 70kHz at the high end,

and down to 2.8Hz in the bass, –3dB. The 1dB

down-points were at 4Hz and 34kHz, giving

a normalised frequency response of 4Hz to

34kHz ±0.5dB. You can see from Graph 1 that

the main deviation in level was at the upper

end of the audio spectrum, with the response

rolling off 0.1dB at 10kHz, to -0.35dB at

20kHz. At the opposite end of the spectrum,

the response was down just 0.05dB at 20Hz

and 0.2dB at 10Hz. Note that Newport Test

Labs decided to use a ‘worst-case’ 20dB gain

setting of the Black Cube, as this involves the

most circuitry. You could expect even betterperformance if you can get away with using

the 10dB or, even better, 0dB gain settings on

the Black Cube.

Newport Test Labs again used the ‘worst-

case scenario’ for channel separation. The fre-

quency response of the left channel is graphed

and runs across the 0dB reference point at the

very top of the graph, though it’s only just

visible due to the vertical scaling, which in

Graph 2 is 9dB per vertical division, compared

to just 0.2dB per vertical division in Graph 1!

You can see that channel separation starts at

84dB and remains at around this level until

it starts diminishing at 700Hz to be 81dB at

Graph 1: Frequency response of line input at 2V output when driven by 500mV input, using

20dB gain setting. [Lehmann Black Cube Linear USB]

Newport Test Labs

Graph 2: Channel separation 500mV input/2 volts output. [Lehmann Black Cube

Linear USB]

Newport Test Labs

Graph 3: Intermodulation distortion (CCIF-IMD) using test signals at 19kHz and

20kHz, referenced to 500mV Input/2 volts output, 20dB gain. [Lehmann Black Cube

Linear USB]

Newport Test Labs

Graph 4: Total harmonic distortion (THD) at 1kHz using 20dB gain setting with

500mV input and 2 volt output. [Lehmann Black Cube Linear USB]

Newport Test Labs

Graph 5: Total harmonic distortion (THD) at 1kHz using 20dB gain setting with

500mV input and 500mV volt output. [Lehmann Black Cube Li near USB]

Newport Test Labs


“You could expect even better performance ifyou can get away with using the 10dB or, even

better, 0dB gain settings on the Black Cube.”




TEST RESULTS


1kHz and 62dB at 20kHz. This is clearly supe-

rior performance

Graph 3 is highly significant, because itshows that intermodulation distortion (in

this case, CCIF-IMD) is vanishingly low. The

two signals that should be there are at 19kHz

and 20kHz. There are high-frequency side-

bands, at 18kHz and 21kHz, but they’re both

at –95dB, equivalent to 0.001% distortion,

but these are not so important as the lack of

regenerated difference signal at 1kHz. There is

a very small signal, as you can see, but it’s at

–105dB—far too low to be of any significance

at all.

Ordinary harmonic distortion varies

a little with gain settings and voltages, but

is always very low. Newport Test Labs tested

many variations to reach a ‘worst-case’ result,

which was in this case the 20dB setting with

a 500mV input and a 2 volt output. You can

see in Graph 4 that there are second and third

harmonic distortion components at –102dB

(0.002%), a fourth harmonic at –113dB and

a sixth at –120dB. Such low components

would not be audible, but even if they were,

the even-order harmonics (second, fourth

and sixth) would tend to complement the au-

dio signal, rather than detract from it.

The final graph (Graph 5) using the same

20dB gain setting and 500mV input as Graph 4,but this time the gain has been reduced to

unity, so the output is also 500mV. You can see

the distortion components have largely disap-

peared, with just second-order and fourth-or-

der harmonics at –110dB (0.001%). You should

note that on both graphs, the tiny blip in the

noise floor just to the left of the 16kHz graph

line is the line frequency of a near-by computer

monitor, and so should be ignored.

Channel balance of the Lehmann Black

Cube was measured at 0.4dB, with overall

THD+N at 0.001%. Signal-to-noise ratio, mea-

sured using the 20dB gain setting, 500mV in-

put and referenced to 2V output, came in at

93dB unweighted, improving to 100dB with

IHF A-weighting. The gain settings were very

slightly different to specification with the lab

measuring 0.28dB low for the ‘0dB’ setting,

0.61dB high at the ‘10dB’ setting and 0.28dB

low at the ‘20dB’ setting. These differences

are so slight that measurement error could be

a factor, and are immaterial in any case, since

these are used only for ‘rough’ set-up, just set-

ting the stage for the volume control to do

its stuff.

Although there is no stand-by mode, the

fact that the Lehmann Black Cube Linear drawsa miserly 9.36-watts should encourage you to

leave it on continuously, because it won’t im-

pact on your power bill and will certainly im-

prove performance in the long run.

Steve Holding

Lehmann Audio Black Cube Linear USB - Test Results

Test Measured Result Units/Comment

Frequency Response @ 1 watt 4.0Hz–34kHz –1dB

Frequency Response @ 1 watt 2.8Hz–70kHz –3dB

Channel Separation 84dB/82dB/64dB (20Hz/1kHz/20kHz)

Channel Balance 0.04dB @ 1kHz

THD+N 0.001% 20dB Gain Setting

S/N Ratio (unweighted) (20dB Gain Setting) 93dB dB re 0.5Vin/2V out

S/N Ratio (weighted) (20dB Gain Setting) 100dB dB re 0.5Vin/2V out

Input Sensitivity (20dB gain) 00.0mV/000mV For 2V out

Maximum Gain at 0dB/10dB/20dB Settings -0.28/10.61/19.72 dB

Power Consumption 9.36 watts

Mains Voltage Variation 238–250 volts Min–Max


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